I've got somewhere in the range of $150.00 to $200.00 in my conversion. To do that I had to start with a Raleigh Technium frame that I stole from my daughter and I already had a significant stash of parts so I didn't have to buy much. I built the wheels myself with new Surly hubs but I already had the rims and spokes. I bought the front brake and cross lever new, but I had access to wholesale prices for bike parts.

If you like to tinker with mechanical things a fixed conversion can be a good project that I highly recommend for the experience. I wouldn't plan on it being a money saver.